1. Field of the Invention
The present invention relates to a turbomachine and, more particularly, to a turbomachine which is designed to prevent conditions giving rise to positively-sloped head-capacity characteristics, which would otherwise be observed in the head-capacity curve while the machine operates below maximum capacity, or which exhibits the positively-sloped head-capacity characteristics only over a portion of its capacity, whereby the turbomachine has a stable operation.
2. Description of the Related Art
FIGS. 3(a) and 3(c) each show an impeller part of a respective conventional turbomachine. FIG. 3(a) shows the impeller part of a turbomachine having an open impeller without a front shroud, while FIG. 3(c) shows the impeller part of the turbomachine having a closed impeller with a front shroud. FIGS. 3(b) and 3(d) are sectional views taken along lines C--C and D--D in FIGS. 3(a) and 3(c), respectively. As is illustrated in the figures, as an impeller 1 rotates inside a casing 3 about an axis 2 of rotation, a fluid is sucked into the casing 3 from a suction port (not shown) and is discharged into a discharge port (not shown).
In the conventional turbomachinery of the type described above, a large-scale separation of flow occurs due to an unstable high-loss fluid, that is, a low-momentum fluid, on the blade surface, the casing and/or the shroud. As a result, a head-capacity curve having a positive slope appears in a partial capacity range, as shown by the broken line 9 in FIG. 6. Such positively-sloped characteristics of the head-capacity curve are also known as a stall phenomenon, which may induce surge, that is, self-induced vibration of a turbomachine piping system, and which may also cause vibration and noise and damage the apparatus. Thus, the stall phenomenon is a serious problem to be solved in obtaining a stable operation of turbomachinery.
The means for solving such a problem may be roughly divided into passive means that are not supplied with energy from the outside of the turbomachine, and active means that are supplied with some energy from the outside of the turbomachine.
Known passive means include casing treatment in which grooves are provided in the inner wall of the casing, and an annular passage with straightening vanes provided inside a part of the casing at the impeller inlet part (see the teaching material for the 181st course sponsored by the Kansai Branch of the Japan Society of Mechanical Engineers, pp. 45-56). These means suffer, however, from the problem that although the effectiveness of the turbomachinery during operation in a partial capacity range is enhanced, the efficiency during the normal operation is accordingly lower.
Further, a means which bypasses fluid from the discharge side toward the inlet side during operation in the partial capacity range is widely employed. However, this means increases the actual capacity of the fluid flowing through the turbomachine, and it inevitably causes a marked reduction in the pump head of the turbomachine. In addition, since a large amount of fluid flows back through the bypass, a great deal of power is consumed disadvantageously.
On the other hand, the conventional active means may be roughly divided into the following four types:
(1) Means for externally supplying energy to the low-momentum fluid on the blade surface, the casing and/or the shroud; PA1 (2) Means for removing such a low-momentum fluid; PA1 (3) Means for imparting a prerotation to the impeller inlet flow, rotating in the direction of the impeller rotation, to thereby prevent blade stalling; and PA1 (4) Means for actively generating disturbances to dump a wave mode of unstable fluid oscillation that appears in the flow field before stalling occurs.
As one example of the means (1), Japanese Patent Application Public Disclosure No. 55-35173 (1980) discloses a method in which part of the high-pressure side fluid is introduced to the tip of the impeller and/or the area inbetween each pair of adjacent blades in the form of a high-speed jet. According to this literature, the jet may be injected in the radial direction, the direction of rotation of the impeller or the direction counter to the impeller rotation, and this literature claims that the jet is equally effective when injected in any of these three directions. Since the function of the jet in this prior art is to supply energy to the unstable low-momentum fluid on the blade surface and to thereby prevent boundary-layer separation, the direction of injection need not particularly be taken into consideration.
As another known example, Japanese Patent Application Public Disclosure No. 45-14921 (1970) discloses a method in which high-pressure air is taken out from the discharge side of a centrifugal compressor and is jetted out from a nozzle provided in a part of the casing that covers the rear half of the impeller to thereby stabilize the pump while operating at partial capacity. The function of the jet in this prior art is to create a turbine effect whereby pressure is supplied to the low-pressure region at the rear part of the blade (blade suction surface side), and a jet flap effect whereby the effective passage width at the impeller exit is reduced. Accordingly, the jet needs to have a circumferential velocity component in the direction of the impeller rotation and also a velocity component in the direction perpendicular to the casing wall surface.
As one example of the means (2), Japanese Patent Application Public Disclosure No. 39-13700 (1964) discloses a means by which a fluid is returned from the high-pressure stage side to the low-pressure stage side in an axial flow compressor to draw low-momentum fluid, which is present inside the boundary layer, along the casing wall at the high-pressure stage side, thereby stabilizing the flow. In this prior art, the return fluid in the low-pressure stage acts as a jet so as to supply momentum to the fluid in the vicinity of the wall surface, thereby also providing the same function as that of the above-described means (1).
As one example of the means (3), Japanese Patent Application Public disclosure No. 56-167813 (1981) discloses an apparatus for preventing surface in a turbo-charger, in which air is injected from an opening facing tangentially to the direction of rotation of the impeller at the impeller inlet. It is stated in this literature that the function of the injected air is to impart a prerotation to the flow so as to reduce the angle of attack of the flow relative to the blades, thereby preventing separation on the blade surface. Accordingly, the direction in which the air is injected is defined as being the same as the direction of rotation of the impeller and tangential to it. This necessitates imparting a prerotation to the flow over a relatively wide range of the blade height in order to prevent stalling over a significant range of partial capacity of the pump and inevitably results in a reduction in the pressure head.
As one example of the means (4), UK Patent Application GB 2191606A discloses a method in which an unstable, fluctuating wave mode in the flow field is measured and, while doing so, the amplitude, phase, frequency, etc. of the wave mode are analyzed, and a vibrating blade, vibrating wall, an intermittent jet, etc. are used as an actuator to actively impart to the fluid such a wave disturbance as to cancel the above-described unstable wave mode, thereby preventing stalling, surge, pressure pulsation, etc. This method is based on the assumption that there is an unstable wave motion as a precursor of stall, surge, etc., and hence cannot be applied to turbomachinery in which such a wave motion is not present.